Photovoltaic (PV) solar cells convert sunlight directly into electricity, and it is desirable to improve the efficiency of a PV solar cell through the particular materials and structure employed. Conventional prior art single junction cells such as those shown in FIGS. 1A and 1B apply a single intrinsic layer to convert sunlight into electricity. FIG. 1A is a single junction nanocrystalline (nc-Si) solar cell and FIG. 1B is a single junction a-Si solar cell. The single junction nc-Si solar cell 100 of FIG. 1A includes a p+ layer 102, a n+ layer 106, and a nc-Si:H layer 104 deposited there between. The nc-Si:H layer 104 has a thickness T1A of approximately 1 to 10 μm. Similarly, the conventional single junction a-Si solar cell 110 of FIG. 1B includes a p+ layer 112, an a-Si:H layer 114 having a thickness T1B of approximately 400 nm, and a n+ layer 116. These solar cells comprised of one layer of active material (nc-Si:H or a-Si:H). These cells, having a single optical bandgap are sensitive only to a limited range of photon energies. Thus, photons with energies below the optical bandgap energy are not absorbed at all. Moreover, the energy of photons in excess of the optical bandgap energy is dissipated as heat in the solar cell. In order to increase the efficiency of energy conversion of solar cells, it is desirable to match the energy gap of the material of the solar spectrum. Therefore, the ability to select the energy gap of the material for a solar cell structure maximizes the efficiency of the solar cell.
The efficiency can be increased by cascading active material into a multijunction solar cell, so that each active layer is responsive to a different region of the solar spectrum. For example, refer to U.S. Pat. Nos. 4,017,332, 4,179,702, and 4,255,211. This approach has several technical weaknesses such as limited material availability, device complexity, current matching and high cost to manufacture. Also, as the current output of a series connected solar cell is limited by the sub cell generating the least amount of photo current. The performance of multi junction device is highly susceptible to air-mass index dependent difference in the solar spectrum.
It is therefore desirable to provide a semiconductor device that includes materials which overcome the problems described herein while improving efficiency of the semiconductor device.